The prairie vole is a five-percenter. When a male prairie vole mates, something happens to his brain. He tends to stay near her, even when other females are around, and then helps out with the kids when they arrive--grooming them, huddling around them to keep them warm, and so on. By contrast, the meadow vole, a close relative, is a ninety-five percenter. Male meadow voles typically couldn't care less. They're attracted to the scent of other females and don't offer parental care.

Scientists have searched for years now for the molecular basis for this difference. One promising candidate was a molecule called the vasopressin V1a receptor (V1aR). In certain parts of the brains, male prairie voles produce more V1aR than meadow voles. To test whether this difference had anything to do with the dedication of male meadow voles, Larry Young of Emory University and his colleagues injected a virus carrying the V1aR gene into the brains of medow voles. As they report today in Nature, the virus caused the meadow voles to begin huddling with their mates almost as loyally as prairie voles.